1. Field of the Invention
The present invention relates to a back-illuminated image sensor wherein energy rays (visible light, ultraviolet rays, soft X-rays, electron beams, and so on) are received at one plane side of a semiconductor (second-plane side) and optoelectronically converted signal charges are transported to a charge transfer unit on the other plane side (first-plane side) for readout.
2. Description of the Related Art
FIG. 36 is a diagram showing a conventional back-illuminated image sensor 601.
In FIG. 36, a semiconductor base 602 consisting of a P-type epitaxial layer is formed in a thickness of the order of 10 μm. An N-type CCD diffusion layer 603 is formed in the first-plane side of the semiconductor base 602 for the sake of charge transfer. The CCD diffusion layer 603 has a plurality of transfer electrodes 605 arranged thereon across a gate oxide film 604. The semiconductor base 602 is provided with an antireflection film 609, a support substrate 611, and so on.
The image sensor 601 configured thus is illuminated at its second-plane side with energy rays. The energy rays produce electron-hole pairs on the second-plane side of the semiconductor base 602. Some of the electrons travel through the semiconductor base 602 until they reach a potential well of the CCD diffusion layer 603 and are accumulated as a signal charge.
The signal charge in the CCD diffusion layer 603 is transferred by applying voltages to the transfer electrodes 605, and is successively read out to exterior.
In such a back-illuminated image sensor 601, the electrons produced at the second-plane side travel through the semiconductor base 602. Accordingly, there has been a problem that the electrons tend to recombine with holes and disappear, with a low efficiency of energy ray detection.
There has been another problem that the electrons traveling through the semiconductor base 602 get mixed between pixels, and thereby cause smear. The smear increases with a decreasing pixel pitch of the image sensor. On that account, the smear production makes it extremely difficult to improve the image sensor in resolution.
In particular, energy rays of shorter wavelengths, such as ultraviolet rays, produce electron-hole pairs at very shallow depth in the second-plane side. Therefore, in the cases of short-wavelength energy rays, the above-mentioned two problems tend to be more significant since electrons have particularly longer traveling distances.
Meanwhile, there has been another problem that the characteristics of the image sensor 601 largely depend on the impurity concentration and thickness of the semiconductor base 602. For example, when the semiconductor base 602 varies in impurity concentration and thickness at the time of manufacturing, the frequency of the electron-hole recombination varies from one product to another, resulting in varying efficiency of the energy ray detection. Additionally, the degree of mixing of electrons between adjoining pixels also varies from one product to another, so that smear occurs in different degrees. Such product variations have been a major cause for the image sensor 601 to drop in production yield.
Furthermore, in the conventional image sensor 601, electrons continually flow in from the second-plane side even while the signal charge is being transferred. This has required that the second-plane side be completely shielded from light during charge transfer. Thus, there has been a problem that the conventional image sensor 601 requires a mechanical shutter, which complicates the peripheral mechanisms of the image sensor.
In addition, the conventional image sensor 601 has had a problem that many dark currents arise at the interface between the antireflection film 609 and the semiconductor base 602, and at the interface between the gate oxide film 604 and the CCD diffusion layer 603. Such dark currents have caused unwanted effects including a deterioration in imaging quality and the impossibility of weak light detection.
Moreover, in the conventional image sensor 601, the surface state and trapped charges create potential wells near the surface of the second plane (hereinafter, referred to as “backside wells”). The backside wells capture electrons and lower the efficiency of the energy ray detection.
By the way, for the sake of fabricating the conventional image sensor 601, it is required that the transfer electrodes 605 and others be formed from the first-plane side, and that holes intended for bonding pads be made from the second-plane side. Conventionally, the alignment of such double-sided structure is achieved by using special devices such as a double side aligner or an infrared aligner.
More specifically, in the cases of a double side aligner, a structure was formed on the second-plane side while the alignment was made to an alignment mark on the first-plane side.
In the cases of an infrared aligner, the alignment mark on the first-plane side was sensed through from the second-plane side by using an infrared ray. By using the perspective image of the alignment marks as an alignment reference, the structure on the second-plane side was formed.
Double side aligners, however, produce alignment errors of ±2 μm. Infrared aligners also produce alignment errors of ±3 μm.
The alignment errors are ascribable to the indirect use of the alignment mark on the opposite plane. Thus, it has been extremely difficult in principle to improve the precision of the alignment. Accordingly, there has been a problem that whichever aligner is used, double-sided structure cannot be aligned with precision.